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Cracks Appearing When Cup Used for First Time


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Puzzling new experience for me.  Two cups -- both from the same firing -- I sold to a lady cracked upon first use -- when she poured hot liquid in them.  Cracks were jaggedly vertical -- almost invisible but allowing leaks from the cups.  They had appeared to be fine when they came out of the kiln -- perhaps thinner than most but I was pleased -- not worried -- about that fact. 

Anyone care to venture some ideas why this would happen?  I'm now worried about others that may have failed -- or will fail --without my knowledge.

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I will try to post pic tomorrow.  As to "poor glaze fit" I assume that means a poor matching of glaze with that particular clay?  It was a clay/glaze combo that is my standard with a long history of success.  But re the "unvitrified body" --  might that be from a firing that got interrupted by a power outage?

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21 minutes ago, Rick Wise said:

I will try to post pic tomorrow.  As to "poor glaze fit" I assume that means a poor matching of glaze with that particular clay?  It was a clay/glaze combo that is my standard with a long history of success.  But re the "unvitrified body" --  might that be from a firing that got interrupted by a power outage?

Poor fit would mean that the glaze and clay body have incompatible coefficients of expansion.  Meaning on a graph where the x axis is expansion and y axis is temperature, at room temperature they may be close enough, but as temperature increases, the expansions differ too much and it crazes.

As far as the unvitrified body, an interrupted or incomplete firing can cause both the leaky body AND a crazing glaze.

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I'm reading the description as a crack in the pots and not crazing, is this correct?  @Rick Wise, do you have any more of the cups with the same glaze and approximate thinness of the ones you sold? If you do, freeze them overnight in the coldest part of your freezer then put them in the sink and pour boiling water into them and see if the same thing happens. I'm wondering if you have some dunting going on if the body of the cup has a fine crack along with the glaze. You mentioned these cups were made thinly, when the glaze thickness is greater than the wall thickness you can get dunting. Where glaze can pool at the bottom of the pot, especially if it has a straight wall meeting a flat base (rather than a rounded base following up into a curved wall),  can cause dunting on a thin pot. Poor glaze fit will exacerbate this. 

 

Edited by Min
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Interesting. We could use more information.

For instance, I raw glaze, and in testing wether I could pour in and pour out with a liner glaze, I managed to put a crack straight thru a piece, in quite the same fashion. Thing is, if I wasn't sitting there watching the pot, I would have totally missed the crack, as after it dried, it shrank back down and you can't even tell. But I garauntee that teapot is garbage now.

I was burned with a pot of coffee on my lap as a child, at the Blue Roof Place. So I personally, wouldn't let anything that may contain hot liquid go without many tests.

Sorce

Edited by Sorcery
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Oh wow!  That's quite something!

According to hamer and hamer, this type of crack in glazed ware 

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[This crack] in glazed ware, especially dense stoneware.  There is stress between the body and glaze which cannot be absorbed.  The body is compressing the glaze but the glaze is strong and does not flake away as in [shivering].  The glaze remains one piece because it is well attached to a dense and finely-grained body.  The body therefore ruptures and as it does so there is sideways displacement o strain to accomodate the stress.  This displacement ruptures the brittle glaze also.

The entry then refers you to dunting.

Under dunting there are 3 types, 2 which occur inside of the kiln, and one which occurs "up to one or two months after removing from kiln".

This type which is what I assume you are suffering, is known as a "cristabolite dunt".  But they warn that a similar stress can occur just from differing contraction rates between clay and glaze.

 

I hope that helps you solve your issue!  Sounds like either way it's a clay body issue (too much unfused free silica), or a clay body and glaze issue.

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